Gas generators typically generate a low density gas from a high density material, such as a liquid or solid, by vaporizing the liquid or solid and/or decomposing or combusting the liquid or solid. In doing so, the resulting generated gas is typically very hot (in the case of decomposition or combustion) or excessively cold (in the case of vaporization of a liquid having a high vapor pressure at room temperature). Very hot gases can melt or damage the object that one is attempting to inflate. Very cold gases will absorb heat during the time after inflation and their volume/pressure will increase, causing explosion or damage of the object being inflated if the exact quantity of required cold gas is not calculated and regulated before inflating.
U.S. Pat. No. 4,627,822 to Esposito discloses a low temperature inflator assembly having a liquid carbon dioxide cartridge and a solid pyrotechnic gas generator. When the inflator is activated, combustion gas supplies immediate low pressure inflation while heat transferred from the generator to the cartridge accelerates vaporization of the carbon dioxide. There are many problems with Esposito. First, once activated, the inflator is completely uncontrollable. Therefore, the total volume of gas needed must be known before the inflator is even built, and its size must be adjusted accordingly. Second, the inflator is usable only a single time, because the solid pyrotechnic gas generator will decompose or combust until it is spent. Third, the temperature of the gas generated by the inflator has a varying time dependence, because the hot gas from the solid pyrotechnic gas generator is released first and the cooler evaporated carbon dioxide is released later, after being heated by heat conduction from the hot solid pyrotechnic gases.
Like conventional gas generators, rocket engines suffer from an exhaust gas that is too hot or too cold. A simple gas thruster, which is simply a container of high-pressure gas with a valve and nozzle, creates thrust by expanding the gas through the nozzle in one direction. Due to the Joule-Thomson and other effects, the expanded gas is excessively cold, and the performance and specific impulse of such a system is very low. To increase specific impulse, a typical combustion-type rocket engine combusts reactive substances to create very hot, high-pressure gases and expands these gases through a nozzle in one direction. These hot gases are typically much higher than the melting point of the nozzle, so that either regenerative, ablative, or other cooling of the nozzle is necessary. Such nozzles are expensive, and a slight miscalculation can result in explosion of the nozzle. Further, in addition to the risk of explosion, such rocket engines are very unsafe because of the hot gases being ejected. While amateur and model rocketry currently utilizes relatively safe solid propellant rocket engines, such engines still create dangerous hot gases.